The present invention relates to a BTL amplifying system for driving a load such as a speaker.
The present application claims priority from Japanese Application No. 2002-054034, the disclosure of which is incorporated herein by reference.
Conventionally, a BTL (Balanced Transformer Less) amplifier has been known as an amplifier for driving a load such as a speaker. The BTL amplifier is able to directly drive a load such as a speaker, and therefore, not only it excels in a frequency characteristic and a distortion characteristic, but also it offers an excellent advantage that a large output can be obtained on a low source voltage. For this reason, it has been used extensively in a portable audio device or in an audio device mounted on a vehicle, operating on a limited source voltage using a battery or the like.
FIG. 8 is a view showing a conventional BTL amplifying system for driving a speaker, and it shows a case of using a BTL amplifier 1 in the form of an IC (integrated circuit) to serve as a direct-coupled amplifier (also referred to as a DC-coupled amplifier).
The BTL amplifier 1 receives an input audio signal having its amplitude adjusted in an electronic volume circuit 2, such as a variable attenuator, through a coupling capacitor 3, and is provided with a non-inverting amplifying circuit 4 for applying non-inverting amplification to the input audio signal, and an inverting amplifying circuit 5 for applying inverting amplification to the input audio signal. Further, it is provided with a power amplifying circuit 6 of a B class push-pull type for applying power amplification to the audio signal having its voltage amplified in the non-inverting amplifying circuit 4, and another power amplifying circuit 7 of a B class push-pull type for applying power amplification to the audio signal having its voltage amplified in the inverting amplifying circuit 5. A speaker 8 is connected somewhere between the output terminals of the power amplifying circuits 6 and 7.
According to this configuration, the power amplifying circuits 6 and 7 are supplied with phase-inverted audio signals from the non-inverting amplifying circuit 4 and the inverting amplifying circuit 5, respectively. Hence, when the power amplifying circuit 6 supplies a power-amplified audio output current to the speaker 8, the power amplifying circuit 7 operates so as to draw a current from the speaker 8. On the other hand, when the power amplifying circuit 7 supplies a power-amplified audio output current to the speaker 8, the power amplifying circuit 6 operates so as to draw a current from the speaker 8.
Given Vcc as the source voltage of the power amplifying circuits 6 and 7 and RL as the load of the speaker 8, then the maximum output power Pout is approximately (Vcc)2/(2xc3x97RL), which is approximately four times of (Vdd)2/(8xc3x97RL), that is approximately the maximum output power of a SEPP (Single Ended Push-Pull) amplifier. This makes it possible to form an audio device or the like capable of obtaining a large output on a low source voltage.
Incidentally, because the conventional BTL amplifying system described above is composed of the direct-coupled BTL amplifier 1 in the form of an IC to serve as an integrated circuit device, it is configured in such a manner so as to prevent an input of a DC component superimposed on the input audio signal when the amplitude is adjusted in the electronic volume circuit 2. As a result, only an AC input audio signal is inputted through the coupling capacitor 3.
However, in the event that leakage occurs due to deterioration of the coupling capacitor 3 or the characteristic of the BTL amplifier 1 varies due to the influence of an environment, a DC offset current may be outputted to the speaker 8 from the BTL amplifier 1. This causes a problem that a heat generated when the offset current flows through a voice coil in the speaker 8 damages the speaker 8.
The present invention was devised in view of the foregoing problems, and therefore, it is an object of the present invention to provide an improved BTL amplifying system capable of preventing an offset from being applied to a load such as a speaker, and thereby forestalling damage to the load.
In order to achieve the above and other objects, a first aspect of the invention provides a BTL amplifying system provided with two power amplifying circuits of a BTL configuration for driving a load, the system comprising: detecting device for detecting a change in output voltages to be supplied to the load from the respective power amplifying circuits; and judging device for judging an occurrence of an offset from the change in the output voltages detected by the detecting device. In particular, the detecting device and the judging device continue detecting and judging operations while the power amplifying circuits are driving the load, and driving of the load by the power amplifying circuits is stopped when the judging device judges the occurrence of an offset.
According to the BTL amplifying system configured as above, the occurrence of an offset is detected and judged while the power amplifying circuits of the BTL configuration are driving the load, and the driving operation is stopped upon detection and judgment of the occurrence of an offset. Hence, even when an offset occurs while the load is driven due to an influence of an environment or due to a deterioration caused by the passing of time in an electronic element incorporated as one of the components, it is possible to forestall offset-induced adverse affects or damage to the load.
A second aspect of the invention is characterized in that the detecting device includes: difference device for calculating a difference between the output voltages to be supplied to the load from the respective power amplifying circuits; and comparing device for comparing the difference calculated by the difference device with a certain threshold, and generating a signal indicating a comparison result of the difference with respect to the certain threshold. In particular, the judging device judges the occurrence of an offset from a characteristic of a change in the signal indicating the comparison result and generated by the comparing device.
According to the BTL amplifying system configured as above, a signal for detecting the occurrence of an offset is found by calculating the difference between the output voltages to be supplied to the load from the respective power amplifying circuits, and the occurrence of an offset is judged based on the characteristic of a change in the signal indicating a comparison result obtained by comparing the difference with the certain threshold.
A third aspect of the invention is characterized in that the detecting device includes comparing device for comparing one of the output voltages to be supplied to the load from the respective power amplifying circuits with a certain threshold, and generating a signal indicating a comparison result of one of the output voltages with respect to the certain threshold. Specifically, the judging device judges the occurrence of an offset from a characteristic of a change in the signal indicating the comparison result and generated by the comparing device.
According to the BTL amplifying system configured as above, one of the output voltages to be supplied to the load from the respective power amplifying circuits is compared with the certain threshold, and the occurrence of an offset is judged from the characteristic of a change in the signal indicating the comparison result of the output voltage with respect to the certain threshold.
A fourth aspect of the invention is characterized in that judging device judges the occurrence of an offset from a voltage value of a smoothed signal obtained through smoothing device for smoothing the signal indicating the comparison result and generated by the comparing device.
According to the BTL amplifying system configured as above, the signal indicating the comparison result and generated by the comparing device is converted to a smoothed signal, and the occurrence of an offset is judged based on the voltage value of the smoothed signal.
A fifth aspect of the invention is characterized in that the judging device judges the occurrence of an offset from an information about a measurement result obtained through measuring device for measuring a change with time in a portion having larger magnitude and a portion having smaller magnitude of the signal indicating the comparison result and generated by the comparing device.
According to the BTL amplifying system configured as above, a characteristic for use in judging the occurrence of an offset is found by measuring a change with time in a portion having larger magnitude and a portion having smaller magnitude of the signal indicating the comparison result and generated by the comparing device.
A sixth aspect of the invention is characterized by further including turn-OFF device for stopping the driving of the load by stopping amplifying operation of the respective power amplifying circuits in response to a judgment of the occurrence of an offset made by the judging device.
A seventh aspect of the invention is characterized by further including turn-OFF device for stopping the driving of the load by inhibiting signal input to the respective power amplifying circuits in response to a judgment of the occurrence of an offset made by the judging device.
According to each of the sixth and seventh aspects of the invention, when an offset occurs, an operation of driving the load is stopped by stopping the amplifying operation of the respective power amplifying circuits or inhibiting a signal to be amplified from being inputted to the respective power amplifying circuits.
An eighth aspect of the invention is characterized by further including device for presenting a warning in response to a judgment of the occurrence of an offset made by the judging device. According to this configuration, it is possible for a user or the like to receive a warning signal indicating that an offset has occurred and driving of the load has been stopped.